西遊记 (The Journey to the West)
On the way to the West, the team came to a Y-junction, either go Left or Right.
The Monkey King (孙悟空 ) went out reconnaissance, only came back few days later.
The Pig (猪八戒) suspects this fellow is a monster in disguise of the monkey, may not tell the truth.
The smart Monk (三藏) asks only 2 questions to detect his identity :
Q1. Is ‘Left’ direction correct ?
Q2. If I ask you Q1, you would say “Yes”, would-you?
The true Monkey will answer “Yes/Yes” or “No/No”, regardless the correct direction.
Case 1. If the Correct direction =Left
Monkey: (Q1, Q2) = (Y,Y)
Monster (tell lie) = (N,Y)
Case 2. If the Correct direction =Right
Monkey : =(N,N)
Monster (tell lie) =(Y, N)